The invention described herein relates to nuclear reactors and more particularly to apparatus for controlling the reactivity during operations.
Conventional commercial nuclear power reactors contain fuel in a sufficient size and configuration which enables the reactor to continuously sustain a fission-type chain reaction. Initially, at start up, the amount of fuel in the reactor core is always greater than that needed for the chain reaction in order to obtain operation over a long time period and to compensate for fissile material burn-up and the generation of reaction products resulting from the fission process. Since the excess fuel produces more neutrons than needed to sustain the chain reaction, the excess neutrons must be controlled to maintain operation at a criticality point and thereby prevent the reactor from reaching an uncontrolled condition, conventionally, such control is primarily accomplished by absorbing neutrons in non-fissioning material and permitting the escape of others from the reactor. Axially movable control rods absorb neutrons and although the control rods may perform separate control shimming and safety functions, stationary rods of neutron absorbing material also may be utilized for absorbing the excess neutrons. Liquid moderating coolants such as water, or water containing boron, for example, also are used to assure reactor operation within prescribed ranges and under certain control conditions.
Neutrons generated by fission move at high velocities and therefore at different energy levels and fission in the reactor is primarily caused by neutron possessing thermal energies, i.e., neutrons at or near thermal equilibrium with the surrounding medium. Since light water reactors depend on thermal neutrons to produce the major portion of their fissions, it is important to moderate fast neutrons to bring them to engery levels more acceptable to the fissioning process. In a light water reactor, moderation is accomplished by utilizing ordinary water, circulating through the reactor core. At the beginning of life, at start-up when excess neutrons are thus generated, the moderating capabilities of the reactor can be lessened and this action will cause fewer neutrons born in fission to reach thermal energy. The neutrons which do not reach thermal energy are utilized to convert fissionable U238 to fissile Pu239. As the fissile material decreases during extended reactor operation, the amount of moderation can be increased to provide additional slowing of the neutrons and still sustain the required reactivity level. However, the apparatus and material needed to carry out changes in reactor moderation is costly, even it may provide a uniform or gradual change in the moderating material.